The principle governing the operation of most magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve where the resistance increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are a seed layer on which is an antiferromagnetic layer whose purpose is to act as a pinning agent for a magnetically pinned layer. Next is a copper spacer layer on which is a low coercivity (free) ferromagnetic layer. A contacting layer lies atop the free layer. When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay in a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field.
If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers suffer less scattering. Thus, the resistance in this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to its two surfaces. However, as the quest for ever greater densities has progressed, devices that measure current flowing perpendicular to the plane (CPP) have also been developed.
A related device to the CPP GMR described above is the magnetic tunneling junction (MTJ) in which the layer that separates the free and pinned layers is a non-magnetic insulator, such as alumina or silica. Its thickness needs to be such that it will transmit a significant tunneling current. The principle governing the operation of the MTJ cell in magnetic RAMs is the change of resistivity of the tunnel junction between two ferromagnetic layers. When the magnetizations of the pinned and free layers are in opposite directions, the tunneling resistance increases due to a reduction in the tunneling probability. The change of resistance is typically 40%, which is much larger than for GMR devices.
To fabricate GMR or MTJ read heads, liftoff technology is commonly used as part of the process, particularly at the step where the readable track width gets defined. Liftoff masks of the prior art are formed out of two distinct and separately deposited layers. Both layers are photo-sensitive but the lower layer is readily removed by one or more solvents (that do not react with other materials present) while the upper layer is resistant to chemical attack.
As the critical dimensions of liftoff patterns grow smaller, it becomes increasingly difficult to make liftoff resists that have the required resolution. A conventional single layer resist cannot be used for liftoff because it is necessary for the upper (etch resistant) layer to overhang the lower (easily etched) layer. Without such overhang, coating of the resist sidewalls by deposited material cannot be avoided so selective removal of the lower layer becomes difficult or impossible.
As track-widths decrease to less than 100 nm, the amount of undercut becomes a large fraction of the total pattern width, making the resist structure mechanically unstable and liable to collapse. Production of a small amount of undercut by a developer solvent becomes very difficult to control. As a result, the use of dual-layer resists for lift-off are becoming impractical as track-widths continue to decrease.
The present invention discloses a method of liftoff based on the use of a single layer of resist.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 6,519,124, Redon et al. disclose a conventional lift-off resist process while in U.S. Pat. No. 5,212,044 Liang et al. describe a method of hardening a resist using an ion beam.